Abstract: The latest progress and development prospects of high-safety
lithium-ion battery research are reviewed. This article mainly introduces the
causes and mechanisms of thermal instability of lithium-ion batteries from the
high-temperature stability of electrolytes and electrodes, clarifies the
shortcomings of existing commercial lithium-ion battery systems at high
temperatures, and proposes the development of high-temperature electrolytes,
positive and negative electrode modifications, and External battery management,
etc. to design high-safety lithium-ion batteries.
Abstract: The latest progress and development prospects of high-safety
lithium-ion battery research are reviewed. This article mainly introduces the
causes and mechanisms of thermal instability of lithium-ion batteries from the
high-temperature stability of electrolytes and electrodes, clarifies the
shortcomings of existing commercial lithium-ion battery systems at high
temperatures, and proposes the development of high-temperature electrolytes,
positive and negative electrode modifications, and External battery management,
etc. to design high-safety lithium-ion batteries. The technical prospects of
developing safe lithium batteries are prospected.
0 Preface
Lithium-ion batteries have become a typical representative of a new type of
energy due to their low cost, high performance, high power, green environment
and many other advantages. They are widely used in 3C digital products, mobile
power supplies, power tools and other fields. In recent years, due to the
intensification of environmental pollution and the guidance of national
policies, the demand for lithium-ion batteries in the electric transportation
market, mainly electric vehicles, has continued to increase. In the process of
developing high-power lithium-ion battery systems, battery safety issues have
attracted widespread attention. , the existing problems urgently need to be
further solved.
The temperature change of the battery system is determined by two factors:
heat generation and dissipation. Heat generation in lithium-ion batteries is
mainly caused by thermal decomposition and reactions between battery materials.
Reduce the heat of the battery system and improve the high temperature
resistance of the system, and the battery system will be safer. Unlike small
portable devices such as mobile phones and laptops, whose battery capacity is
generally less than 2 Ah, the power lithium-ion battery capacity used in
electric vehicles is generally greater than 10 Ah. During normal operation, the
local temperature is often higher than 55°C, and the internal temperature can
reach 300°C. Above, under high-temperature or high-rate charge and discharge
conditions, the heat release of high-energy electrodes and the increase in the
temperature of flammable organic solvents will cause a series of side reactions
to occur, eventually leading to thermal runaway and battery combustion or
explosion [3]. In addition to thermal runaway caused by its own chemical
reaction factors, some human factors such as short circuit caused by
overheating, overcharging, and mechanical shock can also cause thermal
instability of lithium-ion batteries and cause safety accidents. Therefore, it
is of great practical significance to study and improve the high-temperature
performance of lithium-ion batteries.
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1 Analysis of causes of thermal runaway
Thermal runaway of lithium-ion batteries is mainly caused by the increase
in the internal temperature of the battery. Currently, the most widely used
electrolyte system in commercial lithium-ion batteries is a mixed carbonate
solution of LiPF6. This type of solvent has high volatility, low flash point,
and is very easy to burn. When an internal short circuit is caused by collision
or deformation, a large amount of heat will be generated, causing the battery
temperature to rise. When a certain temperature is reached, a series of
decomposition reactions will occur, destroying the thermal balance of the
battery. When the heat released by these chemical reactions cannot be evacuated
in time, the reaction will be accelerated and trigger a series of self-heating
side reactions. The temperature of the battery rises sharply, which is called
"thermal runaway", eventually causing the battery to burn and even explode in
severe cases.
In general, the causes of thermal runaway in lithium-ion batteries mainly
focus on two major aspects: the thermal instability of the electrolyte and the
thermal instability of the coexistence system between the electrolyte and the
positive and negative electrodes.
At present, from a large perspective, safety lithium-ion batteries mainly
take measures from two aspects: external management and internal design, and
control internal temperature, voltage, and air pressure to achieve safety
purposes.
2 Strategies to solve thermal runaway
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2.1 External management
1) PTC (Positive Temperature Coefficient) element: PTC elements are
installed in lithium-ion batteries, which comprehensively consider the pressure
and temperature inside the battery. When the battery heats up due to
overcharging, the internal resistance of the battery increases rapidly, thereby
limiting the current, causing the positive The voltage between the negative
electrodes drops to a safe voltage, realizing the automatic protection function
of the battery.
2) Explosion-proof valve: When the internal pressure of the battery is too
high due to abnormality, the explosion-proof valve deforms and cuts off the
leads placed inside the battery for connection, stopping charging.
3) Electronic circuit: Battery packs of 2 to 4 cells can be pre-embedded
with electronic circuits to design lithium-ion protectors to avoid overcharge
and over-discharge, thereby avoiding safety accidents and extending battery
life.
Of course, these external control methods have certain effects, but these
additional devices increase the complexity and production cost of the battery,
and cannot completely solve the battery safety problem. Therefore, it is
necessary to establish an inherent security protection mechanism.
2.2 Improve the electrolyte system
The electrolyte is the blood of lithium-ion batteries. The properties of
the electrolyte directly determine the performance of the battery, and play an
important role in the battery's capacity, operating temperature range, cycle
performance and safety performance. Currently, the most widely used components
of commercial lithium-ion battery electrolyte systems are LiPF6, ethylene
carbonate and linear carbonate. The first two are indispensable components, and
their use also creates certain limitations in battery performance. At the same
time, a large number of carbonate solvents with low boiling point and low flash
point are used in the electrolyte, which will break down at lower temperatures.
Flashover is a huge safety hazard. Therefore, many researchers have tried to
improve the electrolyte system to improve the safety performance of the
electrolyte. As long as the main battery materials (including electrode
materials, separator materials and electrolyte materials) do not undergo
disruptive changes in a short period of time, improving the stability of the
electrolyte is an important way to enhance the safety of lithium-ion
batteries.
2.2.1 Functional additives
Functional additives have the characteristics of low dosage and strong
pertinence. That is, certain macroscopic properties of the battery can be
significantly improved without increasing or basically increasing the cost of
the battery or changing the production process. Therefore, functional additives
have become a research hotspot in the field of lithium-ion batteries today and
are one of the most promising ways to solve the current flammability problem of
lithium-ion battery electrolytes. The basic function of the additive is to
prevent the battery temperature from being too high and limit the battery
voltage within a controllable range. Therefore, the design of additives is also
considered from the perspective of temperature and charging potential.
Flame retardant additives: Flame retardant additives can be divided into
organophosphorus flame retardant additives, nitrogen-containing compound flame
retardant additives, halogenated carbonate flame retardant additives, silicon
flame retardant additives and composite flame retardant additives according to
different flame retardant elements. 5 main categories.
Organophosphide flame retardants: mainly include some alkyl phosphates,
alkyl phosphites, fluorinated phosphates and phosphazene compounds. The flame
retardant mechanism is mainly the chain reaction of flame retardant molecules
interfering with hydroxyl radicals, also known as the free radical capture
mechanism. The vaporization and decomposition of the additive releases
phosphorus-containing free radicals, which have the ability to capture hydrogen
radicals in the system to terminate the chain reaction.
Phosphate ester flame retardants: mainly include trimethyl phosphate,
triethyl phosphate (TEP), tributyl phosphate (TBP), etc. Phosphazenes such as
hexamethylphosphazene (HMPN), alkyl phosphites such as trimethyl phosphite
(TMPI), tris-(2,2,2-trifluoroethyl), phosphite (TT- FP), fluorinated phosphates
such as tris-(2,2,2-trifluoroethyl)phosphate (TFP),
bis-(2,2,2-trifluoroethyl)-methylphosphate (BMP) ,
(2,2,2-trifluoroethyl)-diethylphosphate (TDP), phenyloctylphosphate (DPOF), etc.
are all good flame retardant additives. Phosphate esters usually have relatively
high viscosity and poor electrochemical stability. The addition of flame
retardants not only improves the flame retardancy of the electrolyte, but also
has a negative impact on the ionic conductivity of the electrolyte and the cycle
reversibility of the battery. The solutions are generally: ① increase the carbon
content of the alkyl group; ② partially replace the alkyl group with an aromatic
(phenyl) group; ③ form a phosphate ester with a cyclic structure.
Organohalogenated substances (halogenated solvents): Organohalogenated
flame retardants mainly refer to fluorinated organic substances. When H in
non-aqueous solvents is replaced by F, its physical properties will change, such
as lower melting point, lower viscosity, and improved chemical and
electrochemical stability. Organic halogenated flame retardants mainly include
fluorinated cyclic carbonates, fluorinated chain carbonates and
alkyl-perfluoroalkyl ethers. OHMI and other studies comparing fluorinated ethers
and fluorinated esters with fluorinated compounds have shown that the 0.67 mol/L
LiClO4/EC+DEC+PC (volume ratio 1:1:1) electrolyte with 33.3% (volume fraction)
of fluorinated compounds added has the It has a higher flash point and a higher
reduction potential than organic solvents EC, DEC and PC. It can quickly
generate an SEI film on the surface of natural graphite, improving the Coulombic
efficiency and discharge capacity of the first charge and discharge.
Fluorine itself does not have the free radical trapping function like the
flame retardants mentioned above, but only plays the role of diluting highly
volatile and flammable co-solvents. Therefore, it can only be used when its
volume ratio in the electrolyte accounts for the majority. (>70%), the
electrolyte is non-flammable.
Composite flame retardants: Composite flame retardants currently used in
electrolytes include P-F compounds and N-P compounds. Representative substances
mainly include hexamethylphosphoramide (HMPA), fluorinated phosphates, etc.
Flame retardants exert their flame retardant effect through the synergistic
effect of two flame retardant elements.
FEI et al. proposed two N-P flame retardants, MEEP and MEE, whose molecular
formulas are shown in Figure 1. LiCF3SO3/MEEP:PC=25:75, the electrolyte can
reduce the flammability by 90%, and the conductivity can reach 2.5 × 10-3
S/cm.
2) Overcharge additive: When a lithium-ion battery is overcharged, a series
of reactions will occur. The electrolyte components (mainly solvents) undergo
irreversible oxidation and decomposition reactions on the surface of the
positive electrode, producing gas and releasing a large amount of heat, which
leads to an increase in the internal pressure and temperature of the battery,
seriously affecting the safety of the battery. In terms of mechanism of action,
overcharge protection additives are mainly divided into two types: redox shuttle
type and electropolymerization type. In terms of additive types, they can be
divided into lithium halides and metallocene compounds. The overcharge additives
currently used in large-scale applications mainly include biphenyl (BP) and
cyclohexylbenzene (CHB). For redox anti-overcharge additives, the principle is
that when the charging voltage exceeds the normal cut-off voltage of the
battery, the additives begin to generate energy in the positive electrode.
Oxidation reaction, the oxidation product diffuses to the negative electrode,
and a reduction reaction occurs. The redox couple shuttles between the positive
and negative electrodes, absorbing excess charge. Its representative substances
include ferrocene and its derivatives, complexes of 2,2-pyridine and
1,10-phenanthroline of ferrous ions, and thianthrene derivatives.
Polymerization blocking additive to prevent overcharge. Representative
substances include cyclohexylbenzene, biphenyl and other substances. When
biphenyl is used as an anti-overcharge additive, when the voltage reaches 4.5 ~
4.7 V, the added biphenyl polymerizes electrochemically, forming a conductive
film on the surface of the positive electrode, which increases the internal
resistance of the battery, thereby limiting the charging current and protecting
the battery. .
2.2.2 Ionic liquids
Ionic liquid electrolytes are composed entirely of anions and cations.
Because the anions or cations are large in size, the interaction between anions
and cations is weak and the electron distribution is uneven. Anions and cations
can move freely at room temperature and are in a liquid state. Generally
speaking, they can be divided into imidazoles, pyrazole and pyridine, quaternary
ammonium salts, etc. Compared with ordinary organic solvents for lithium-ion
batteries, ionic liquids have five main advantages: ① High thermal stability and
no decomposition at 200°C; ② The vapor pressure is almost 0, so there is no need
to worry about bloating in the battery; ③ Ionic liquids are not flammable ,
non-corrosive; ④ high electrical conductivity; ⑤ good chemical or
electrochemical stability.
AN et al. formulated an electrolyte solution of PP13TFSI and 1 mol LiPF6
EC/DEC (1:1), which can achieve a completely non-flammable effect. Adding 2wt%
LiBOB additive to this system can also significantly improve the interface
compatibility.
The only problem that remains to be solved is the conductivity of ions in
the electrolyte system.
2.2.3 Select lithium salt with good thermal stability
Lithium hexafluorophosphate (LiPF6) is the electrolyte lithium salt widely
used in commercial lithium-ion batteries. Although its single property is not
optimal, its comprehensive performance is the most advantageous. But LiPF6 also
has its shortcomings. For example, LiPF6 is chemically and thermodynamically
unstable and will undergo the following reactions:
LiPF(6s)→LiF(s)+PF(5g)
The PF5 generated by this reaction can easily attack the lone pair of
electrons on the oxygen atom in the organic solvent, leading to ring-opening
polymerization of the solvent and cleavage of the ether bond. This reaction is
particularly serious at high temperatures.
Current research on high-temperature electrolyte salts is mostly focused on
the field of organic lithium salts. Representative substances mainly include
boron lithium salt and imide lithium salt. LiB(C2O4)2 (LiBOB) is a newly
synthesized electrolyte salt in recent years. It has many excellent properties,
has a decomposition temperature of 302°C, and can form a stable SEI film on the
negative electrode. Improves the performance of graphite in PC-based
electrolyte, but its viscosity is high and the resistance of the SEI film formed
is large [14]. The decomposition temperature of LiN(SO2CF3)2(LiTFSI) is above
360°C, its ionic conductivity at room temperature is slightly lower than that of
LiPF6, it has good electrochemical stability, and its oxidation potential is
about 5.0 V. It is the most studied organic lithium salt. But it severely
corrodes Al-based current collectors.
2.2.4 Polymer electrolyte
Many commercial lithium-ion batteries use flammable and volatile carbonate
solvents, which may cause fires if leakage occurs. This is especially true for
power lithium-ion batteries with large capacity and high energy density. The use
of non-flammable polymer electrolytes instead of flammable organic liquid
electrolytes can significantly improve the safety of lithium-ion batteries.
Research on polymer electrolytes, especially gel polymer electrolytes, has
made great progress. It has been successfully used in commercial lithium-ion
batteries. According to the main polymer classification, gel polymer
electrolytes mainly include the following three categories: PAN-based polymer
electrolyte, PMMA polymer electrolyte, and PVDF-based polymer electrolyte.
However, gel polymer electrolytes are actually the result of a compromise
between dry polymer electrolytes and liquid electrolytes. Gel polymer batteries
still have a lot of work to do.
2.3 Cathode material
It can be determined that the cathode material is unstable when the charged
state voltage is higher than 4 V, and is prone to thermal decomposition at high
temperatures to release oxygen. Oxygen and organic solvents continue to react to
generate a large amount of heat and other gases, reducing the safety of the
battery [2,17- 19]. Therefore, the reaction between the positive electrode and
the electrolyte is considered to be the main cause of thermal runaway.
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