Liquid electrolytes have reached their research limits, and many scientists
are now focusing on solid electrolytes. But Professor Meng Ying, director of the
Center for Sustainable Power and Energy and the Energy Storage and Conversion
Laboratory at the University of California, San Diego, led her team to go in the
opposite direction, studying gaseous electrolytes and making breakthroughs.
These gaseous electrolytes can liquefy under a certain pressure and are more
resistant to freezing. In the new research, they selected two liquefied gases -
fluoromethane and difluoromethane - from a large number of gas candidates to
make electrolytes for lithium batteries and supercapacitors respectively,
extending the minimum operating temperature of lithium batteries from minus 20
to The operating temperature of supercapacitors extends from minus 40 degrees
below zero to minus 80 degrees below zero. Moreover, these electrolytes can
still work efficiently after returning to normal room temperature. In addition
to setting low-temperature operating records, these gaseous electrolytes also
overcome the common thermal runaway problem in lithium batteries and have
greater safety advantages. Thermal runaway is a vicious cycle of heat in the
battery. When the battery is working, the temperature will rise, starting a
series of chemical reactions. The heat generated by these reactions will in turn
further heat the battery, causing the battery to expand and be destroyed.
1. In actual product production, these indicators are often contradictory,
and battery-related performance needs to be weighed and considered. Improving
battery performance requires taking into account the performance of electrode
materials, electrolytes, and separators. At the same time, the follow-up of
assembly technology, battery system grouping, and management technology are also
crucial. This article aims to summarize the current development achievements of
power batteries with lithium-ion batteries as the core from aspects such as
battery material technology, single battery design and manufacturing technology,
and battery system technology, while looking forward to the future!
1. Lithium battery material technology positive and negative electrode
materials Lithium battery positive and negative material systems are very rich.
At present, research on positive electrode materials such as lithium cobalt
oxide, lithium manganate, lithium iron phosphate, and lithium nickel cobalt
manganese has become mature. The specific capacity of lithium cobalt oxide
material is 200-210mA·h/g. Its material true density and electrode plate
compacted density are the highest among existing cathode materials. The charging
voltage of commercial lithium cobalt oxide/graphite system can be increased by
4.40V. It can already meet the demand for high-volume energy-density soft-pack
batteries for smartphones and tablets. Lithium manganate has low raw material
cost, simple production process, high thermal stability, good overcharge
resistance, high discharge voltage platform and high safety. It is suitable as a
low-cost battery for light electric vehicles, but it has problems such as
relatively low theoretical capacity, and the possible dissolution of manganese
during the cycle, which affects the life of the battery in high-temperature
environments. Domestic lithium manganate materials mainly meet the needs of the
mobile power supply, power tools and electric bicycle markets, and have a
tendency to develop towards the low end.
NCM ternary layered cathode materials are mainly used in power batteries.
In addition to LiNi1/3Co1/3Mn1/3O2, which each accounts for 1/3 of nickel,
cobalt, and manganese, its application in power batteries is relatively mature.
LiNi0.5Co0 with higher capacity .2Mn0.3O2 has also entered batch applications
and is generally used in electric vehicle batteries mixed with lithium
manganate. The energy density of aluminum-doped lithium nickel cobalt oxide
(NCA) can be close to high-voltage lithium cobalt oxide batteries. In recent
years, electric vehicle manufacturer Tesla has used this computer battery to
drive electric vehicles. The material can also be used with lithium manganate
Hybrids are used to manufacture vehicle power batteries. Domestic NCA precursors
have formed stable production capacity. A few companies have completed the
development of NCA cathode materials and are in the process of product
promotion.
Lithium iron phosphate batteries have high safety and long life. At
present, nanoscale power materials and high-density lithium iron manganese
phosphate materials are developing rapidly. The performance of high-energy and
high-power materials tends to be stable, and the cost is further reduced.
Gradually meeting the needs of the domestic market and the promotion of new
energy vehicles in China at this stage, high-voltage spinel lithium nickel
manganese oxide and high-voltage high-specific capacity lithium-rich
manganese-based cathode materials are still under development. Negative
electrode materials The negative electrode materials that can be used in power
batteries include graphite, hard/soft carbon and alloy materials. Graphite is
currently a widely used negative electrode material, and its reversible capacity
can reach 360mA·h/g.
Amorphous hard carbon or soft carbon can meet the needs of batteries for
higher rate and lower temperature applications and is beginning to be used, but
it is mainly mixed with graphite. Lithium titanate anode material has optimal
rate performance and cycle performance and is suitable for high-current
fast-charging batteries, but the battery produced has low specific energy and
high cost. Nano-silicon was proposed to be used in high-capacity anodes in the
1990s. Doping a small amount of nano-silicon to increase the capacity of carbon
anode materials is a current research and development hotspot. Anode materials
that add a small amount of nano-silicon or silicon oxide have begun to enter
small batches. In the application stage, the reversible capacity reaches
450mA·h/g.
However, due to the volume expansion caused by lithium being embedded in
silicon, the problem of reduced cycle life during actual use needs to be further
solved. Electrolyte Lithium-ion battery electrolyte is generally a mixture of
high dielectric constant cyclic carbonate and low dielectric constant linear
carbonate. Generally speaking, the electrolyte of lithium-ion battery should
meet the requirements of high ionic conductivity (10-3~10-2S/cm), low electronic
conductivity, wide electrochemical window (0~5V), and good thermal stability
(-40~60℃ ) and other requirements. Lithium hexafluorophosphate and other new
lithium salts, solvent purification, electrolyte preparation, and functional
additive technology continue to advance. The current development direction is to
further increase its operating voltage and improve the high and low temperature
performance of the battery. Safe ionic liquid electrolytes and solid
electrolytes are under development. Separator Polyolefin microporous membrane is
currently the main variety in the lithium-ion battery separator market due to
its excellent mechanical properties, good electrochemical stability and
relatively low cost [7].
Including polyethylene (PE) single-layer film, polypropylene (PP)
single-layer film and PP/PE/PP three-layer composite microporous film. There are
many manufacturers in China that use the dry process for production, and there
are many companies that can mass-produce wet process PE separators. As ceramic
coating technology is promoted, high temperature and high voltage resistant
diaphragms will become the future research and development direction.
2. Single cell technology So far, the basic design of lithium-ion batteries
is still the same as that announced by SONY in 1989. The shapes of single cells
include cylindrical, square metal shells (aluminum/steel) and square soft packs.
The original cylindrical batteries Mainly used in laptops, 18650 cylindrical
batteries are now used in electric vehicles by companies such as Tesla.
Square batteries generally have a larger capacity, and the cells are
produced by rolling, Z-shaped lamination, winding + lamination, positive
electrode coating lamination, lamination + winding, etc. Cylindrical battery
cell technology is the most mature and has lower manufacturing costs. However,
large cylindrical batteries have poor heat dissipation capabilities, so small
cylindrical batteries are generally used. Vehicle battery packs have a large
capacity and a large number of batteries, and the management system is complex
and expensive. Among square batteries, the production process of the winding
structure battery is relatively simple, but it is mainly suitable for soft
electrode sheet batteries. This method can be used for batteries using lithium
iron phosphate and ternary materials in addition to spinel cathode
materials.
Laminated batteries with high reliability and long life are suitable for
various material systems. The batteries of GM Volt plug-in hybrid electric
vehicle and Nissan Leaf pure electric vehicle are manufactured using the
lamination process. By 2015, the specific energy of lithium iron phosphate
single cells will reach 140W·h/kg, and the specific energy of ternary material
mixed lithium manganate single cells will reach 180W·h/kg. The specific energy
of small cylindrical batteries using NCA internationally is Reaching 240W·h/kg,
the specific energy of lithium-ion single cells will further increase in the
next few years, and is expected to reach a maximum of 300W·h/kg by 2020.
3. Battery system technology From the perspective of commercialized
lithium-ion power battery systems, key core technologies include battery pack
technology (integrated battery pack, thermal management, collision safety,
electrical safety, etc.), battery management system (BMS) electromagnetic
compatibility Technology, accurate measurement of signals (such as cell voltage,
current, etc.) technology, accurate estimation of battery status, battery
balance control technology, etc. The BMS and other key core components of the
battery system, including sensors, controllers, actuators and other components,
are basically monopolized by the automotive electronics technology powers
(Germany, Japan, and the United States).
At present, some domestic companies have successfully developed smart
meters, which can replace foreign current, voltage, and insulation sensors. The
primary factor affecting the promotion and application of electric vehicles is
the safety and cost of use of lithium-ion power batteries. In addition to
further improvements in the safety, lifespan and consistency of the battery
itself, battery modular technology, battery group technology (integrated battery
pack, Thermal management, collision safety, electrical safety, etc.) also have
obvious gaps with foreign countries. At present, the battery pack technology of
international automobile companies is relatively mature, and domestic research
units have conducted relatively in-depth research on BMS electromagnetic
compatibility technology, precise signal measurement technology, accurate
battery status estimation, and battery balance control technology.
The research and development of key battery power management technologies
include comprehensive battery electrochemical models, electrical safety design,
battery state estimation, equalization management, fault diagnosis and
calibration, and charging management. The research and development of key
battery thermal management technologies and systems need to study the heat
dissipation and temperature equalization effects of different thermal management
technologies based on the structural design of the battery pack and battery heat
generation calculation and analysis, and obtain a battery thermal management and
cooling solution with low cost, simple process, strong safety and reliability
.
To reduce the weight of the battery structure, we need to take the battery
system and the related structure of the vehicle as the research object, consider
the coupling characteristics between each other, and carry out integrated
optimization of structural vibration resistance, impact resistance and
lightweight from two aspects: structural design optimization and material
selection. Design key technology research work. Optimize component materials,
structural design, connection and other design plans. In terms of battery
safety, it is necessary to carry out overall safety plan design research on the
battery system based on electrical safety, mechanical safety and thermal safety,
and carry out fault diagnosis and prediction for the battery system. , thermal
safety monitoring, early warning and key technologies for prevention and
control.
4. Looking forward to a long period of time in the future, lithium-ion
batteries will still be the most suitable electric vehicle batteries, including
lithium manganate cathode materials, ternary system cathode materials, lithium
iron phosphate cathode materials, composite carbon anode materials, and ceramic
coating separators. The development of , electrolyte salt and functional
electrolyte technology supports the progress of battery technology and
industrial development.
As battery system technology advances in application, safety and
reliability will be further improved in the coming years. Research on the life
model and model influencing parameters of lithium-ion power batteries, research
on battery grouping characteristics, research on balancing strategy of
high-efficiency and large-capacity lithium-ion batteries, single battery charge
and discharge thermal model and group battery pack temperature field analysis
and control methods Research, research on optimized fast charging methods for
group batteries needs to be carried out.
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