(1Institute of Nuclear and New Energy Technology, Tsinghua University,
Beijing 100084)
Abstract: This article reviews the development history of lithium-ion
battery cathode material production and preparation technology, and analyzes the
development direction of lithium-ion battery cathode materials. At the end of
the last century, from the perspective of the processing performance of
lithium-ion battery cathode materials and battery performance, a research team
from Tsinghua University proposed a technology to control crystallization to
prepare high-density spherical precursors. Combined with the subsequent
solid-phase sintering process, they proposed a method for preparing
lithium-containing electrode materials. industrial technology. Among them, the
controlled crystallization method to prepare precursors can regulate and
optimize the properties of the material at four levels: unit cell structure,
primary particle composition and morphology, secondary particle size and
morphology, and particle surface chemistry. The materials produced using this
technology have the characteristics of easy control of particle size and
morphology, good uniformity, batch consistency and stability, and can
simultaneously meet the comprehensive requirements of the battery for the
electrochemical performance and processing performance of the material. Due to
the high packing density of the material, it is especially suitable for high
specific energy batteries. This technology is suitable for a variety of cathode
materials and is suitable for mass production. Over time, it has been gradually
proven to be the best production technology for cathode materials for
lithium-ion batteries, and has been generally accepted and recognized by the
industry today. This is also one of the important contributions made by Chinese
scientists to the international lithium-ion battery industry.
Keywords: lithium-ion battery; cathode material; production technology;
controlled crystallization; development direction
Energy Storage Science and Technology, 2018, 7(5): 888-896
Lithium-ion batteries have the advantages of high specific energy, high
energy storage efficiency and long life. In recent years, they have gradually
occupied the main market share of electric vehicles, energy storage systems and
mobile electronic devices. Since the Japanese company Sony took the lead in
commercializing lithium-ion batteries in 1990, the negative electrode material
has always been carbon-based materials, while the positive electrode material
has made great progress and is the most critical material to promote the
performance improvement of lithium-ion batteries.
The research and development of cathode materials for lithium-ion batteries
is mainly carried out in three aspects: 1) The basic science level, mainly the
discovery of new materials, or the calculation, design and synthesis of material
composition, crystal structure and defect structure, with a view to discovering
electrode materials. New cathode materials with excellent chemical properties;
2) At the material chemistry level, the synthesis technology is mainly discussed
in order to optimize the material structure factors such as material crystal
structure, orientation, particle morphology, interface, etc., and obtain
electrochemical performance, processing performance and battery performance.
Best match, the purpose is to develop material structures and synthesis methods
that can optimize the comprehensive performance of cathode materials; 3)
Material engineering technology level, mainly to develop large-scale, low-cost,
stable equipment and processes, in order to develop reasonable Engineering
technology to meet market needs.
In order for lithium-ion battery cathode materials to exert the best
performance in the full battery, it is necessary to further optimize the crystal
structure, particle structure and morphology, particle surface chemistry,
material packing density and compaction density of the material on the premise
of optimizing the material composition. physical and chemical properties, and it
is also necessary to strictly prevent the introduction of trace metal impurities
during the process. Of course, stable, high-quality mass production is an
important guarantee for the stable performance of materials in battery
manufacturing. As lithium battery technology improves and the lithium battery
market matures, the application fields of different cathode materials are
gradually divided, that is, lithium-ion batteries have different performance
requirements for various cathode materials. Therefore, the mainstream synthesis
technologies and processes of cathode materials have also experienced different
development paths.
This article reviews the industrial application history of the main cathode
materials for lithium-ion batteries, focuses on the development of materials'
industrial technology, and looks forward to the development direction of cathode
material manufacturing technology.
1. Performance requirements for cathode materials in lithium-ion
batteries
(1) Industry performance requirements for lithium-ion batteries
To understand the technical indicators of cathode materials, we need to
start with the technical indicators of the battery. In the early days of the
lithium-ion battery industry, it mainly served the development of mobile
electronic products, such as laptops, tablets, mobile smart terminals (mobile
phones), etc. In recent years, the new energy industry and the electric vehicle
industry have risen rapidly, and the demand for lithium-ion batteries has grown
rapidly, stimulating the lithium battery industry to accelerate its development.
Therefore, lithium-ion batteries need to meet many technical performance
indicators in order to be recognized by the industry and achieve further
development. Among these technical indicators, the most basic include specific
energy, cycle stability, specific power, cost, safety and reliability,
durability, manufacturing efficiency, sustainability, etc. The indicators are
interrelated, and different application fields have important Priorities for
lithium-ion battery metrics are different. Compared with lithium-ion batteries
in portable electronic products, the biggest difference between energy storage
and lithium-ion batteries used in the electric vehicle industry is that the
capacity of a single battery has increased by ten or even dozens of times. At
the same time, the function and structure of the battery module And the
complexity of applications has increased significantly, which has put forward
higher requirements for the consistency and reliability of lithium-ion
batteries.
Based on more than 20 years of research and engineering practice
experience, the team of this article believes that the most important technical
indicators of lithium-ion batteries are specific energy and cycle performance,
followed by performance indicators such as specific power, safety, reliability,
cost and consistency. The higher the specific energy, the material cost per unit
energy (Wh) decreases; the longer the cycle life, the lower the actual use cost
of the battery. At present, lithium-ion batteries for mobile smart terminals
need to meet the requirements of specific energy of more than 700 Wh/L and cycle
performance of more than 200 times, while lithium-ion batteries for electric
vehicles need to meet the specific energy of 140 Wh/kg (lithium iron phosphate
or lithium manganate cathode material) or 200 Wh/kg (layered oxide cathode
material) or more, and cycle performance of more than 1,500 times. Lithium-ion
battery cathode materials must meet the above battery indicators before they can
be accepted by the mainstream battery market. At present, the specific energy
and cycle performance of lithium-ion batteries mainly depend on the cathode
material [1-6]. Therefore, the main research and development goals of cathode
materials for lithium-ion batteries are high specific energy and long cycle
life.
For lithium-ion batteries used in laptops, tablets, and mobile smart
terminals, the volume specific energy is the most important indicator. Of
course, batteries with a high volume specific energy usually also have a high
mass specific energy. Because customers want to put more battery energy into
devices of a specific volume (such as mobile phones), currently the graphite |
lithium cobalt oxide system lithium-ion battery is the most mature in
industrialization and has the highest volume specific energy. Lithium in other
material systems It is difficult for ion batteries to shake the dominance of
lithium-ion batteries in this system in the mobile electronics industry. Safety,
reliability and certain cycle performance are also important for this type of
battery. Since it is mainly used in a single form, the consistency and cost of
the battery are not so important.
For lithium-ion batteries used in electric vehicles, although their volume
specific energy requirements are not as stringent as batteries for portable
electronic products, after all, the space of passenger cars is limited, and the
weight of the car body will affect the driving range of the electric vehicle, so
the mass ratio of the battery Energy and Volume Ratio Energy is still very
important. In addition, automotive lithium-ion batteries have almost stringent
performance requirements for almost all other properties, which are much higher
than the performance requirements of batteries for portable electronic products.
There are three biggest differences from batteries for portable electronic
products. First, the power supply of electric vehicles requires higher voltage
and current, and requires a large number of single cells to be combined in
series and parallel. This makes the actual specific energy that can be utilized
by the battery pack not only depend on the specific energy of the single
battery, but also the specific energy of the single battery. Consistency,
especially dynamic consistency, the consistency of power batteries has gradually
attracted people's attention in recent years [7]. Second, the scale of single
cells has increased significantly, which makes the price of single cells higher
and the harm caused by thermal runaway more serious. Therefore, the market is
more sensitive to the safety and reliability of batteries. Third, since electric
vehicles require a service life of 10-15 years, they have very high requirements
for cycle performance, generally requiring more than 1,500 times. In addition,
since electric vehicles need to start and accelerate, the power battery also has
certain requirements for power.
With the rapid development of the electric vehicle industry, power
lithium-ion batteries will become the mainstream products of the lithium battery
industry together with batteries for portable electronic products in the future.
Specific energy and cycle performance are the most important performance
indicators that are always pursued in the development of lithium-ion battery
technology. As safety, reliability, specific power and consistency receive
increasing attention, technology in this area is expected to develop rapidly. It
should be noted that as lithium-ion batteries gradually penetrate into various
fields of the national economy, there will be more and more non-mainstream
lithium-ion battery market segments, which have special requirements for battery
performance indicators and are not discussed in this article. scope.
(2) Cathode materials that meet the needs of the mainstream lithium-ion
battery industry
Currently, the cathode materials that meet the battery performance
requirements of the mainstream lithium-ion battery market mainly include layered
lithium cobalt oxide LiCoO2 material (LCO), spinel lithium manganate LiMn2O4
material (LMO), and olivine lithium iron phosphate LiFePO4 material (LFP) ,
olivine lithium iron manganese phosphate LiMn0.8Fe0.2PO4 material (LMFP),
layered ternary material LiNi1/3Mn1/3Co1/3O2 material (NMC333), layered ternary
material LiNi0.4Mn0.4Co0.2O2 (NMC442), LiNi0.5Mn0.3Co0.2O2 (NMC532),
LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.7Mn0.2Co0.1O2 (NMC721), LiNi0.8Mn0.1Co0.1O2
(NMC811) and layered high-nickel material LiNi0 .8Co0.15Al0.05O2 (NCA), etc.
From the perspective of industrial application, the above materials have
different physical and chemical characteristics and are suitable for lithium-ion
batteries in different application fields. Therefore, the key performance
indicators of the material products are also different.
Lithium cobalt oxide LiCoO2 (LCO) material is currently the cathode
material with the highest compaction density. Therefore, the prepared
lithium-ion battery has the highest volume specific energy and has become the
main cathode material for lithium-ion batteries used in tablet computers and
mobile smart terminals. Its shortcomings are mainly limited cobalt resources and
high cost, which limits its wide application in the field of electric vehicles.
The structure and reaction characteristics of this material are that as the
charging voltage gradually increases, the amount of lithium extracted gradually
increases, and the available capacity of LCO gradually increases. However, when
the amount of lithium extracted exceeds 55% (that is, relative to the charging
potential of metallic lithium) 4.25V (compared to the charging voltage of
graphite | LCO full battery is 4.2V), the structural stability of the material
decreases rapidly, and the lifespan and safety deteriorate rapidly. Therefore,
LCO cathode materials that can withstand higher charging voltages and have
chemical stability that meets battery application requirements are the main
development direction of current material preparation technology. LCO has a
stable structure and is relatively easy to synthesize. Its preparation
technology is simple and relatively mature. Before 2000, LCO was mainly produced
through solid-phase sintering technology of cobalt oxide/lithium carbonate
mixture. With people's ultimate pursuit of product packing density, specific
modification, etc., the method of controlling crystallization to prepare lithium
cobalt oxide precursor has changed. It has the advantage of material morphology
control and has gradually become a major industrial preparation technology
[8-11].
The main advantages of the spinel lithium manganate LiMn2O4 (LMO) material
are abundant raw material resources, low cost, and good battery safety; its main
recognized disadvantages are low battery specific energy and poor cycle
stability. Since the 1990s, attracted by its low raw material and process costs
and good safety, people have explored the application of LMO in electric buses,
passenger cars, special vehicles, power tools and other fields. Traditional
solid-phase sintering preparation technology cannot control the material
structure. In order to improve its cycle stability and the tap density of the
material, the author's team introduced a liquid-phase process to prepare the
precursor in 2004 [12-14], and further used surface coating to Technologies such
as coating, lattice doping, and surface gradient are used to improve material
performance [15-22]. However, due to the high solubility of the material, the
cycle stability of the battery has not been well satisfied. Only by further
combining the electrolyte can the battery life be able to meet the demand. At
present, although LMO is rarely used in vehicle power batteries, it has been
widely used in small power battery industries such as electric bicycles that are
more cost-sensitive. In addition, as people pay attention to the safety of large
power batteries for vehicles, blending with ternary materials has become one of
the main uses of LMO materials.
The main advantages of the olivine lithium iron phosphate LiFePO4 (LFP)
material are abundant raw material resources, low cost, battery safety and good
cycle performance. Its main disadvantage is the low specific energy of the
battery. This material has been widely used not only in the electric bicycles,
electric buses, electric buses, and special vehicle industries, but also in the
large-scale energy storage industry. Since lithium ions in this material are
transported along one-dimensional channels, the material has significant
anisotropy and is extremely sensitive to defect structures. The preparation
process needs to ensure a high degree of uniformity of the synthesis reaction
and a precise Fe:P ratio to achieve better results. capacity and rate
performance. Based on the complexity of the material structure and synthesis
reaction, there are two main problems in the preparation of this material:
First, the process requires a reducing atmosphere. The reaction raw materials
have different requirements for the reducing atmosphere due to different types
and particle sizes. The local reducing properties are too high or too high. If
it is too low, impurities will remain in the product; second, the material needs
to be surface carbon coated or compounded with other types of conductive agents,
which makes it difficult to control the impurities and compaction density of the
material. In 2005, the author's research group proposed to use controlled
crystallization technology to prepare high-performance iron phosphate precursor
(FP), and then prepare LFP through carbothermal reduction with lithium source
and carbon source [11]. The above process route has been further improved and
has become the current mainstream lithium iron phosphate material preparation
technology [23-29]. In order to meet people's continuous pursuit of LFP battery
performance, high uniformity and high batch stability have become the most
concerned product indicators of LFP cathode materials. On the one hand,
traditional solid-phase sintering technology is difficult to achieve efficient
consistency in principle. Control, on the other hand, consistency control can
lead to a significant increase in process costs. Compared with solid-phase
processes, precursors prepared based on liquid-phase processes or cathode
materials prepared based on hydrothermal/solvothermal processes have better
structural adjustability and controllability [30], while batch stability and
reaction Good uniformity. Similar to large chemical plants, continuous
solvothermal processes can easily achieve ultra-large-scale production.
Therefore, liquid phase technology has gradually become the development trend of
the next generation of high-quality LFP cathode material preparation technology
[31-37].
Olivine lithium iron manganese phosphate LiMn0.8Fe0.2PO4 (LMFP) material is
an upgraded version of LFP material, with a specific energy 10% higher than LFP;
due to the difference in the reaction kinetics of Mn and Fe raw materials and
the requirements for reducing atmosphere, this material The main disadvantage is
the difficulty of preparation. At present, the industrial preparation process
based on the solid-phase method is not yet mature and has not yet been applied
on a large scale. If the liquid phase preparation technology of LFP is
industrially applied
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